Need to make a call but run out of juice? Just give your cellphone a shake for an extra burst of power - that's the idea behind a new cellphone charger that turns movement into energy.

Shashank Priya and his colleagues at Virginia Tech in Blacksburg are designing an emergency onboard charger that draws energy from the piezoelectric force generated by your fingertips clicking the keypad, your voice - or just by giving the phone a good old shake.

The researchers experimented with zinc oxide, a common piezoelectric material, to see how well it converts vibrations from sound and pressure waves into energy to power a phone.

They subjected the material to sound waves of 100 decibels, which made the material vibrate and produce an electrical current at about 50 millivolts. In a cellphone, the piezoelectric material would be mounted below the keys and convert mechanical vibrations into energy that could be stored for later use.

It wouldn't produce a great deal of power - certainly not nearly enough to continuously operate a phone - but would be sufficient for an emergency situation, says Priya.

"In an emergency you could just shake your cellphone for a few minutes to get enough power to make this one important call," he says.

For everyday use, the same group recently completed a micro wind-turbine charger that fits into a water-bottle-like container. The charger generates a watt of power with wind speeds of around 8 to 10 kilometres per hour. It connects to a cellphone with a standard USB connection and charging time is normally in the range of 45 minutes to an hour-and-a-half, Priya says.